AIRBORNE is William F. Buckley, Jr.'s description of his sail across the wide Atlantic with his son and five friends. The trip, for 15 years a dream, for 15 months a planned operation, was always a risk. One doesn't set out haphazardly in a small sailboat to cross 4,400 miles of ocean. Buckley's account of perils is, as one might expect, instructive, nonchalant and terrifying.